The present invention relates to a process for continuous digestion at elevated temperature and pressure of finely-divided material by passing said finely-divided material successively through a heating zone, at least one digesting zone and a cooling zone in contact with a liquid phase.
Processes are already well-known in which a finely-divided material, such as wood chips, is digested at elevated temperature and pressure by means of a digesting liquid, such as white liquor, by passing the wood chips from the top downwards through a continuously operating cooking tower divided into different zones for heating and impregnation of the chips, digesting of the heated and impregnated chips and for washing and cooling of the pulp thereby obtained. Such digesting towers are customarily equipped with strainers for extraction of liquid from the solid material at the beginning and end of the zones and possibly also at intermediate positions. The washing liquid has been introduced at the outlet end of the washing zone, near the point at which the pulp is withdrawn, so that the washing liquid flows in counter-current to the pulp in the washing zone. White liquor has been fed into the product liquor withdrawn from the digesting zone in such a manner that the principal direction of flow of the liquid in the digesting zone is either the same as or counter to that of the chips therein.
In processes known heretofore the chips and white liquor have been heated either by directly heating the chips with primary steam and steam obtained from expansion of the black liquor or by mixing the chips with white liquor which is indirectly heated by steam.
Although by means of various improvements to the internal heat recovery in modern continuously operating cookers it has been possible to reduce the heat consumption to approx. 2-3 GJ/tonne of pulp, it has now surprisingly been discovered that a considerable further reduction in the heat consumption can be achieved.
The object of the present invention is thus to achieve a process for the digestion of finely-divided material at elevated temperature and possibly pressure with the use of considerably less heat than heretofore.
According to the present invention there is provided a process for digesting finely-divided material at elevated temperature and pressure wherein finely-divided material and liquid is fed and liquid phase withdrawn from the inlet end of a heating zone in such relative quantities that their heat capacity flows are substantially of the same magnitude.